Insulated glazing units – Practical acoustic considerations
Insulated glazing units (IGUs) are typically selected for their thermal and acoustic properties. On a noisy site, the sound reduction performance of IGUs can be critical to a project’s success. Poorly controlled noise ingress can result in problems such as complaints of disturbance to sleep and rest, and also disrupt work and other important activities.
In acoustics, the sound reduction is normally specified in terms of the weighted sound reduction index (Rw dB), a parameter which is measured in a laboratory. In addition, one would often specify the performance at particular octave band frequencies (eg, R125Hz dB) or a Ctr correction, such that noise sources with certain frequency characteristics are properly controlled.
The laboratory testing of IGUs helps engineers predict how they will work together with other elements, such as lightweight cladding, ventilation openings, masonry walls and so on. If we know how an IGU performs, we can calculate the resulting noise level within a building. Unfortunately, this is not always the case with IGUs which have a very large area, due to how they are tested and issues that can arise with accepted calculation methods for correcting for area.
External windows and doors are subject to British Standard BS EN 14351-1:2006+A2:2016 Windows and doors. Product standard, performance characteristics. The laboratory testing of IGU sound reduction is undertaken according to the BS EN ISO 10140 series (or the superseded BS EN ISO 140 series). All IGUs are tested to the specific dimensions of 1.23 m x 1.48 m (corresponding to a test aperture dimension of 1.25 m x 1.50 m), such that the IGU test area is 1.82 m2. As the area of the IGU increases, the performance of the unit can change, meaning that corrections must be applied to any laboratory test data where the IGU area differs from 1.82 m2.
The sound reduction performance of IGU systems reduces with increased size. The corrections can be extrapolated from BS 14351-1 (see Table 1). These corrections are added to the weighted sound reduction performance (Rw or Rw + Ctr, dB).
This method generally holds true for small differences in the test versus real-world IGU areas. However, it can be less accurate for larger IGU areas because the correction approach only addresses the weighted (single figure) sound reduction performance and does not consider changes in behaviour at individual frequencies.
An IGU behaves a bit like a drum, where the frame is the drum shell, and the glass is the drum skin. When excited by noise hitting the glass, the IGU will have a modal response, like a pitch or tone being heard when hitting a drum. Without getting lost in mathematics, this first tone is the primary mode or ‘natural frequency’ of the IGU, and the second, third, fourth modes and so on are known as the harmonics or modal frequencies. What causes this in IGUs are known as bending waves. Bending waves occur when the bending wavelength (eg, distance) is large compared to the plate thickness. So, thin sheets of material with a relatively large surface area (like a drum) are very good at generating bending waves.
It follows that as IGUs get larger, the greater the modal response. As the IGUs dimensions become comparable to a specific wavelength, the sound reduction at the corresponding frequency will be reduced. The precise frequency at which this happens depends on quite a few factors; including area, glass thickness and density, air cavities, frame conditions etc, and so will differ from IGU to IGU. This makes it hard to predict although finite plate theory can help indicate areas of risk.
These effects have been observed when testing large IGU systems in the laboratory. For example, at one UK test centre, a reduction of around 5-8 dB was observed in the 63-125 Hz octave band frequencies for systems with dimensions in the region of 4.5 m2. In particular, the point at the centre of the glass panels was observed to be radiating noise significantly more than other points. This corresponds to the behaviour of a primary mode resonance shown in Figure 1. The effects can also happen at higher frequencies due to reduced stiffness in the panel or material coincidence dips. For example, on one project, Sandy Brown observed significant losses in performance for an IGU with an area of 4.0 m2 at 500-2000 Hz. This resulted in an overall loss in the region of 6-7 dB in the Rw + Ctr performance.
What we learn from this is that if one applies the British Standard correction approach on a project where the sound reduction performance at specific frequencies is important, one could end up with an IGU that performs on site very differently to expectations. This could result in failure to comply with design criteria, and by extension, planning conditions. Therefore, when considering glazing systems with dimensions notably greater than the laboratory standard, the designer must consider large glazing effects.
A good question to start with is whether a performance dip can be accommodated? If the resulting risk to the project is too great, then there are two good approaches to mitigation. Firstly, one could design in tolerance to the acoustic specification such that even with modal behaviour, internal noise limits are complied with. This could be a costly overdesign to your client however, particularly if the glazing will be extensive. Often the best and most definitive approach is to test the proposed IGU in a laboratory at design size; however, do bear in mind that this is likely to have cost and programme implications.
By Valerie Van den Hende
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